Conventional high-performance computing systems require that the global wiring of the processor have sufficient communication bandwidth, minimal communication delay, lower power consumption, and other features. However, future improvement in the performance of existing metal wiring techniques cannot be sufficiently expected because of the reduced cross-sectional area of wiring, higher drive frequency, and other factors. In view of this situation, on-chip optical wiring techniques based on silicon photonics are promising, but the communication bandwidth needed in wiring continues to increase at a rapid speed, and no matter if optical wiring techniques are used, the bandwidth is insufficient with a single optical wiring layer. Consequently, there are high expectations for three-dimensional optical wiring techniques in which the optical wiring layer is multilayered and which are capable of dramatically increasing wiring density.
Three-dimensional optical wiring techniques not only expand bandwidth, but also have an advantage in that an optical waveguide intersection with low loss and low leakage can be implemented. This is due to the fact that two intersecting optical waveguides can be spatially separated. The importance of an optical waveguide intersection having excellent performance is dramatically increasing in accompaniment with higher density and more complicated optical wiring.
A low-loss optical coupling between two different optical wiring layers is indispensable for implementing three-dimensional optical wiring, and such implementation is not easy. This is because light is different from electricity when there is sudden bending of the transmission channel, or when considerable optical loss occurs due to discontinuity of the refractive index. Therefore, techniques such as the following have been proposed for light wave interchange between layers.
As shown in FIG. 7, a method has been proposed in which a plurality of spot size converters having a tapered structure used mainly in optical signal connection between chip and optical fiber are combined to mutually couple optical signals between optical waveguides disposed in different layers (see non-patent document 1).
In FIG. 7, (a) is a perspective view of an interlayer light wave coupling device, (b) is a schematic plan view thereof, and (c) is a schematic center transverse-sectional view thereof.
The optical waveguides disposed in an upper and lower layer mutually have a tapered structure. Therefore, in the case that optical waveguides of the upper and lower layers each exist independently, there is a location somewhere in the tapered structure in which effective refractive indexes match each other in the upper and lower layers, even when there are structural or material differences in refractive index in each optical waveguide. Accordingly, a light wave coupling that is insensitive to structural or material fluctuations in refractive index can be realized. Additionally, light wave coupling is used and therefore, in principle, results in low loss. Moreover, there is only a single coupling location of light waves between the upper and lower layers, and there is substantially no return of light waves to the source waveguide once the light waves have migrated.
With this method, it is difficult to sufficiently increase the interlayer distance. In other words, the distal end width of the tapered structure is sufficiently narrowed to allow the mode shape of the light waves to widely expand toward the exterior of the optical waveguide. Therefore, light waves can, in principle, be made to couple even when the interlayer distance is sufficiently increased, but in such a case, the light waves readily scatter out from the optical waveguide by slight disturbances in the optical waveguide structure that occur unintentionally, and this results in loss.
In other words, this method is highly efficiency, but has a drawback in that it is difficult to sufficiently increase the interlayer gap.
The next method uses a grating coupler in which a diffraction grating is provided to a side surface of an optical waveguide to thereby allow light to be drawn out to the side surface of the optical waveguide (see patent document 1, and non-patent documents 2 and 3).
However, this method has drawbacks including low efficiency in principle, high wavelength dependency, high dependency on the plane of polarization, and the requirement that specular surfaces disposed in the upper and lower layers be disposed with high positioning precision.
There is also a method that attempts to direct light to an optical waveguide in a different layer by placing a mirror in the optical waveguide to change the direction of propagation of light, and after the light has been directed toward a different layer, a similar mirror is used to direct the light to the optical waveguide of the different layer.
However, it is very difficult to form a specular surface having high reflectivity, and since silicon in particular has a high refractive index, it is even more difficult to form a specular surface.
Furthermore, the positioning tolerance of the specular surface between the upper and lower layers is low, and high manufacturing precision is required (see non-patent document 4).